An energy filter for forming an image is also known as an electron filter, energy filter, or electron spectrometer and used in a transmission electron microscope. The contrast between a pair of imaging portions is improved by selecting electrons lying in a given energy range. Also, an image representing the distribution of elements and a filtered-electron diffraction graph can be recorded. An omega filter is disclosed in U.S. Pat. No. 5,177,361. A gamma filter is disclosed in U.S. Pat. No. 5,585,630.
FIGS. 3(A) and 3(B) show an example of the structure of the omega filter described in the above-cited U.S. Pat. No. 5,177,361. This omega filter is a kind of energy filter. FIG. 3(A) is a cross-sectional view taken on line B--B of FIG. 3(B). FIG. 3(B) is a cross-sectional view taken on line A--A of FIG. 3(A). The omega filter, generally indicated by numeral 1, has two outer yokes 3 and 4. A center yoke 2 is held between these two outer yokes 3 and 4 and provided with a passage 6 shaped like the letter "Q". An electron beam, indicated by 5a or 5b, passes through this passage 6. The two outer yokes 3 and 4 are mounted to the center yoke 2 with bolts 7. Polepieces 8 are firmly mounted to the outer yokes 3 and 4 with bolts 9 and located opposite to four deflection positions along the passage 6. A coil 10 for producing an electromagnetic field is wound around each polepiece 8, thus forming lenses for the omega filter 1. The incident electron beam 5a is deflected by the action of the four lenses, travels in the substantially Q-shaped passage 6, and emerges as the electron beam 5b on the same straight line as the incident electron beam 5a.
Maintaining the desired dimensional accuracy of the length L of the gap between the opposite polepieces 8 shown in FIG. 3(A) is important for the performance of the omega filter. Also, the desired dimensional accuracy of the angle of the incident surface 8a of each polepiece 8 and the angle of the exit surface 8b shown in FIG. 3(B) must be maintained. Furthermore, the incident surface 8a and the exit surface 8b of each polepiece 8 which are opposite to each other should be prevented from being misaligned.
In this conventional omega filter 1, one polepiece 8 is mounted on each of the two outer yokes 3 and 4, together with one coil 10. These outer yokes 3 and 4 are mounted on the center yoke 2, thus forming a three-layered structure. This makes it impossible to check the dimensional accuracies during and after assembly. Moreover, the positions of the individual lenses cannot be readjusted.